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NASA NTRS2003

Physical Characteristics of Asteroid-like Comet Nucleus C/2001 OG108 (LONEOS)

For many years several investigators have suggested that some portion of the near-Earth asteroid population may actually be extinct cometary nuclei. Evidence used to support these hypotheses was based on: observations of asteroid orbits and associated meteor showers (e.g. 3200 Phaethon and the Geminid meteor shower); low activity of short period comet nuclei, which implied nonvolatile surface crusts (e.g. Neujmin 1, Arend-Rigaux); and detections of transient cometary activity in some near-Earth asteroids (e.g. 4015 Wilson-Harrington). Recent investigations have suggested that approximately 5-10% of the near- Earth asteroid population may be extinct comets. However if members of the near-Earth asteroid population are extinct cometary nuclei, then there should be some objects within this population that are near their final stages of evolution and so should demonstrate only low levels of activity. The recent detections of coma from near-Earth object 2001 OG108 have renewed interest in this possible comet-asteroid connection. This paper presents the first high quality ground-based near-infrared reflectance spectrum of a comet nucleus combined with detailed lightcurve and albedo measurements.

Abell, P. A.↗
NASA NTRS2018

Direct Characterization of Comets and Asteroids via Cosmic Dust Analysis from the Deep Space Gateway

The Deep Space Gateway (DSG) may provide a platform for direct sampling of a large number of comets and asteroids, through employment of an instrument for characterizing dust from these bodies. Every year, the Earth traverses through debris streams of dust and small particles from comets and asteroids in Earth-crossing orbits, generating short-lived outbursts of meteor activity commonly known as "meteor showers" (Figure 1). The material in each debris stream originates from a distinct parent body, many of which have been identified. By sampling this material, it is possible to quantitatively analyze the composition of a dozen or more comets and asteroids (See Figure 2, following page) without leaving cislunar space.

Fries, M.↗
NASA NTRS1993

The enigmatic object 2201 Oljato - Is it an asteroid or an evolved comet?

The orbital properties of near-earth object 2201 have been associated with meteor showers, and its modeled orbital evolution is chaotic - a property which might indicate a history related to comets. Telescopic observations of its visible and near-infrared spectral reflectance, broad-band visible and near-infrared photometry, infrared radiometric measurements, and radar echoes are reported here from two apparitions, 1979 and 1983. This asteroid has a high radiometric albedo, a property not associated with comet nuclei. In certain wavelength regimes it is classified as an S-type asteroid, in others, an E-type, but its overall spectral reflectance is not typical of either taxonomic type, and neither type is thought of as cometlike. Unexpectedly high ultraviolet reflectance at the 1979 apparition was suggested to be the result of residual outgassing as in a comet. The UV photometric data are modeled as fluorescent emission from neutral species found in comets. The resulting calculations indicate a plausible value for OH and CN emission at 0.3085 and 0.38 micron relative to the observed range of active comets.

Mcfadden, Lucy A.↗
NASA NTRS1989

Evolution of comets into asteroids

This paper presents observational evidence, together with recent theoretical developments, supporting the hypothesis that at least some asteroids might be extinct or dormant cometary nuclei. The observations include the discovery of a number of apparent asteroids in chaotic Jupiter-crossing orbits; the IRAS discovery of 1983 TB, an asteroid in the same orbit as the Geminid meteor shower; the apparent low activity levels determined for several short-period comet nuclei including Comet Halley; and observations of possible cometary activity in some earth-crossing asteroids. Theoretical developments include explorations of dynamical mechanisms capable of delivering main-belt asteroids into earth-crossing orbits, and an understanding of possible processes which may affect comets during their long residence in the Oort cloud and lead to the formation of nonvolatile crusts before and after they enter the planetary system.

Weissman, Paul R.↗
NASA NTRS2011

Dynamical Model for the Zodiacal Cloud and Sporadic Meteors

The solar system is dusty, and would become dustier over time as asteroids collide and comets disintegrate, except that small debris particles in interplanetary space do not last long. They can be ejected from the solar system by Jupiter, thermally destroyed near the Sun, or physically disrupted by collisions. Also, some are swept by the Earth (and other planets), producing meteors. Here we develop a dynamical model for the solar system meteoroids and use it to explain meteor radar observations. We find that the Jupiter Family Comets (JFCs) are the main source of the prominent concentrations of meteors arriving to the Earth from the helion and antihelion directions. To match the radiant and orbit distributions, as measured by the Canadian Meteor Orbit Radar (CMOR) and Advanced Meteor Orbit Radar (AMOR), our model implies that comets, and JFCs in particular, must frequently disintegrate when reaching orbits with low perihelion distance. Also, the collisional lifetimes of millimeter particles may be longer (approx. > 10(exp 5) yr at 1 AU) than postulated in the standard collisional models (approx 10(exp 4) yr at 1 AU), perhaps because these chondrule-sized meteoroids are stronger than thought before. Using observations of the Infrared Astronomical Satellite (IRAS) to calibrate the model, we find that the total cross section and mass of small meteoroids in the inner solar system are (1.7-3.5) 10(exp 11) sq km and approx. 4 10(exp 19) g, respectively, in a good agreement with previous studies. The mass input required to keep the Zodiacal Cloud (ZC) in a steady state is estimated to be approx. 10(exp 4)-10(exp 5) kg/s. The input is up to approx 10 times larger than found previously, mainly because particles released closer to the Sun have shorter collisional lifetimes, and need to be supplied at a faster rate. The total mass accreted by the Earth in particles between diameters D = 5 micron and 1 cm is found to be approx 15,000 tons/yr (factor of 2 uncertainty), which is a large share of the accretion flux measured by the Long Term Duration Facility (LDEF). Majority of JFC particles plunge into the upper atmosphere at <15 km/s speeds, should survive the atmospheric entry, and can produce micrometeorite falls. This could explain the compositional similarity of samples collected in the Antarctic ice and stratosphere, and those brought from comet Wild 2 by the Stardust spacecraft. Meteor radars such as CMOR and AMOR see only a fraction of the accretion flux (approx 1- 10% and approx 10-50%, respectively), because small particles impacting at low speeds produce ionization levels that are below these radars detection capabilities.

Nesvorny, David↗
NASA NTRS1993

Ultraviolet Imaging Spectrometer

Wide-field imaging systems equipped with objective prisms or gratings have had a long history of utility in groundbased observations of meteors and comets. Deployment of similar instruments from low Earth orbit would allow the first UV observations of meteors. This instrument can be used for comets and Lyman alpha coronae of Earth-orbit-crossing asteroids. A CaF2 prism imaging spectrograph designed for stellar observations was used aboard Skylab to observe Comet Kohoutek (1973f), but its 1300-A cut-off precluded Lyman alpha images and it was not used for observation of meteors. Because the observation of the UV spectrum of a meteor has never been attempted, researchers are denied the opportunity to obtain composition information from spectra at those wavelengths. We propose construction of a flight instrument functioning in the 1100-3200 A spectral range that is suitable for a dedicated satellite ('Quick Star') or as a space-station-attached payload. It can also be an autonomous package in the space shuttle cargo bay.

Wdowiak, T. J.↗
NASA NTRS1986

Precision requirements on cosmic dust trajectory measurements

It has been known for some time that the orbital parameters of certain major meteor streams rather closely match those of presently observed comets. There is therefore a clear parent-daughter orbital relationship between meteoroids in certain streams and the comets that they derived from. For meteoroids in the photographic meteor range, it has been estimated that from 1% to 10% of the meteoroid mass is concentrated into the major streams. However, for the smaller, more numerous meteoroids observed as radar meteors, streams are less intense but there are more of them. Sekanina (1973) has established for the radar meteors that, in addition to comets, some of the parent bodies appear to be asteroids. As noted by Grun et al. (1985), meteoroid lifetimes, due to collisional destruction or Poynting-Robertson (P-R) drag losses, range from 10(5) yr downward to less than 10(3) yr; these meteoroids therefore need to be continuously be replenished by source bodies to maintain the meteoritic complex in some sort of temporal equilibrium. It will be very important to obtain their precise trajectories when meteoroids are collected with a capture apparatus in Earth orbit, as is made apparent with the following logic: a chemical, isotopic, or other analysis of any particular meteoroid constitutes a similarly detailed analysis of a small part of the parent comet or asteroid that is orbitally associated with it. One can, consequently, do rather detailed cometary or asteroid science utilizing only an Earth-orbiting cosmic dust capturing facility.

Zook, H. A.↗
NASA NTRS2015

DIRBE Comet Trails

Re-examination of the COBE DIRBE data reveals the thermal emission of several comet dust trails.The dust trails of 1P/Halley, 169P/NEAT, and 3200 Phaethon have not been previously reported.The known trails of 2P/Encke, and 73P/Schwassmann-Wachmann 3 are also seen. The dust trails have 12 and 25 microns surface brightnesses of <0.1 and <0.15 MJy/sr, respectively, which is <1% of the zodiacal light intensity. The trails are very difficult to see in any single daily image of the sky, but are evident as rapidly moving linear features in movies of the DIRBE data. Some trails are clearest when crossing through the orbital plane of the parent comet, but others are best seen at high ecliptic latitudes as the Earth passes over or under the dust trail. All these comets have known associations with meteor showers. This re-examination also reveals one additional comet and 13 additional asteroids that had not previously been recognized in the DIRBE data.

COMET↗
NASA NTRS2014

Characteristics of the 2012 Geminids

The parent of the Geminids, 3200 Phaethon, is a unique body in that it is classified as an asteroid, however is responsible for one of the most prolific meteor showers of the year and has shown comet-like behavior in its past (Jewitt and Li 2010). The Geminid meteor shower is also anomalous as its rates have been increasing since it was first detected. Understanding the composition and properties of meteoroids that belong to this meteor shower is an important area of study and of interest to both theoreticians and experimentalists. Using the light curve and decelerations of ten double-station Geminids as seen in the Meteoroid Environment Office's widefield meteor cameras, densities were able to be approximated using a model of meteoroid ablation by Campbell-Brown et al (2013) which employs thermal disruption to model the release of grains during ablation. Bulk densities of Geminids give unique insight into the composition of Phaethon that would only be derived by going to the asteroid itself. The bulk densities of these ten Geminids were found to be between 2.6 and 3.0 g/cm(3), supporting results from Babadzhanov (2009) and Borovicka et al (2010) which prove Phaethon has a much lower porosity than most other meteor shower parents. NASA's Meteoroid Environment Office established these two wide-field meteor cameras to observe meteors in the milligram-mass-range. Each camera consists of a 17 mm focal length Schneider lens (f/0.95) on a Watec 902U2 Ultimate CCD video camera, producing a 21.7x15.5 degree field-of-view. This configuration sees meteors down to a magnitude of +6. Data from these cameras are currently being used to calculate daily automated meteor fluxes. On the first night of operation, December 13-14, 2012, 18 double-station and 53 unique single-station Geminids were detected. The Geminid flux results from this system will be presented as well as ZHR's over the peak of the Geminids. The average flux density over the night was 0.058, 0.052, and 0.062 meteors/km(2)/hour down to a limiting magnitude of +6.5, for the double-station results and each single-station's results. This equates to ZHR's of 113, 102, and 122 respectively. Included in the flux algorithm is a process to find the collecting area per height and a method to find the limiting meteor magnitude per 10 minute time period.

Blaauw, R.↗
NASA NTRS2001

In Situ Measurements of Meteoric Ions

Extraterrestrial material is the source of metal ions in the Earth's atmosphere, Each year approx. 10(exp 8) kg of material is intercepted by the Earth. The origin of this material is predominantly solar orbiting interplanetary debris from comets or asteroids that crosses the Earth's orbit. It contains a very small amount of interstellar material. On occasion the Earth passes through enhanced amounts of debris associated with the orbit of a decaying comet. This leads to enhanced meteor shower displays for up to several days. The number flux of shower material is typically several times the average sporadic background influx of material. Meteoric material is some of the earliest material formed in the solar system. By studying the relative elemental abundances of atmospheric metal ions, information can be gained on the chemical composition of cometary debris and the chemical makeup of the early solar system. Using in situ sampling with rocket-borne ion mass spectrometers; there have been approximately 50 flights that made measurements of the metal ion abundances at attitudes between 80 and 130 km. It is this altitude range where incoming meteoric particles am ablated, the larger ones giving rise to visible meteor. displays. In several rocket measurements isotopic ratios of different atomic ion mass components and metal molecular ion concentrations have been determined and used to identify unambiguously the measured species and to investigate the processes controlling the metal ion distributions The composition of the Earth's ionosphere was first sampled by an ion mass spectrometer flown an a rocket in 1956. In 1958 a rocket-borne ion spectrometer identified, fbr the first time, a layer of metal ions near 95 km. These data were interpreted as evidence of an extraterrestrial rather than a terrestrial source. Istomin predicted: "It seems probable that with some improvement in the method that analysis of the ion composition in the E-region may be used for determining the chemical composition of those meteors which do not reach the ground. Particularly, we hope to get information about the composition difference between particles of different meteor showers and also sporadic and shower meteoroids". These visions categorized the aims of many subsequent rocket-borne ion mass spectrometer experiments in the lower ionosphere, Although the use such measurements to deduce the composition of different classes of meteoroids has not been successful, the past four decades of rocket observations have provided po%erful sets of data for advancing our understanding of meteor ablation, meteoric composition, metal neutral and ion chemistry as well as ionospheric dynamics.

Grebowsky, Joseph M.↗
NASA NTRS2018

The Formation and Early Evolution of Meteoroid Streams

Meteor showers occur when the Earth encounters a stream of particles liberated from the surface of a comet or, more rarely, an asteroid. Initially, meteoroids follow a trajectory that is similar to that of their parent comet but modified by both the outward flow of gas from the nucleus and radiation pressure. Sublimating gases impart an "ejection velocity" to solid particles in the coma; this ejection velocity is larger for smaller particles but cannot exceed the speed of the gas itself. Radiation pressure provides a repulsive force that, like gravity, follows an inverse square law, and thus effectively reduces the central potential experienced by small particles. Depending on the optical properties of the particle, the speed of the particle may exceed its effective escape velocity; such particles will be unbound and hence excluded from meteoroid streams and meteor showers. These processes also modify the heliocentric distance at which meteoroid orbits cross the ecliptic plane, and can thus move portions of the stream out of range of the Earth. This talk presents recent work on these components of the early evolution of meteoroid streams and their implications for the meteoroid environment seen at Earth.

Moorhead, Althea V.↗
NASA NTRS1973

The cometary and asteroidal origins of meteors

A quantitative examination of the gravitational and nongravitational changes of orbits shows that for larger interplanetary bodies the perturbations by Jupiter strongly predominate over all other effects, which include perturbations by other planets, splitting of comet nuclei and jet effects of cometary ejections. The structure of meteor streams, indicates that the mutual compensation of the changes in individual elements entering the Jacobian integral, which is characteristic for the comets, does not work among the meteoroids. It appears that additional forces of a different kind must exert appreciable influence on the motion of interplanetary particles of meteoroid size. Nevertheless, the distribution of the Jacobian constant in various samples of meteor orbits furnishes some information on the type of their parent bodies and on the relative contribution of individual sources.

Kresak, L.↗
NASA NTRS2000

Stratospheric Sampling and In Situ Atmospheric Chemical Element Analysis During Meteor Showers: A Resource Study

Resources studies for asteroidal mining evaluation have depended historically on remote sensing analysis for chemical elements. During the November 1998 Leonids meteor shower, a stratospheric balloon and various low-density capture media were used to sample fragments from Comet Tempel-Tuttle debris during a peak Earth crossing. The analysis not only demonstrates how potential sampling strategies may improve the projections for metals or rare elements in astromining, but also benchmarks materials during low temperature (-60 F), high dessication environments as seen during atmospheric exposure. The results indicate high aluminum, magnesium and iron content for various sampled particles recovered, but generalization to the sporadic meteors expected from asteroidal sources will require future improvements in larger sampling volumes before a broad-use strategy for chemical analysis can be described. A repeat of the experimental procedure is planned for the November 1999 Leonids' shower, and various improvements for atmospheric sampling will be discussed.

Noever, David A.↗
NASA NTRS2022

Decadal and Annual Variations in Meteoric Flux from Ulysses, Wind, and SOFIE Observations

Our solar system is filled with meteoric particles, or cosmic dust, which is either interplanetary or interstellar in origin. Interstellar dust (ISD) enters the heliosphere due to the relative motion of the sun and the interstellar flow. Interplanetary dust (IPD) comes primarily from asteroid collisions or comet sublimation, and comprises the bulk of material entering Earth’s atmosphere. This study examines variations in ISD and the IPD flux at Earth using observations from three different satellite techniques. First are size-resolved in situ meteoroid detections by the Ulysses spacecraft, and second are in situ indirect dust observations by Wind. Third are measurements of meteoric smoke in the mesosphere by the Solar Occultation For Ice Experiment (SOFIE). Wind and Ulysses observations are sorted into the interstellar and interplanetary components. Wind ISD show the anticipated correlation to the 22-yr. solar magnetic cycle, and are consistent with model predictions of ISD. Because Wind does not discriminate particle size, the IPD measurements were interpreted using meteoric mass distributions from Ulysses observations and from different models. Wind observations during 2007-2020 indicate a total meteoric influx at Earth of 22 metric tons per day (t d(exp -1)), in reasonable agreement with long-term averages from SOFIE (25 t d(exp -1)) and Ulysses (32 t d(exp -1)). The SOFIE and Wind influx time series both show an unexpected correlation to the 22-yr. solar cycle. This relationship could be an artifact, or may indicate that IPD responds to changes in the solar magnetic field.

Meteoric influx↗
NASA NTRS1978

Interplanetary dust - Possible implications for comets and presolar interstellar grains

Interplanetary dust is important to studies of the origin of the solar system because it is material from comets and asteroids, the smallest surviving bodies from the early solar system. The fact that comets are the major suppliers of the millimeter meteoroids which produce optical meteors suggests that a significant fraction of the submillimeter portion of the meteoroid complex is also cometary material. While comets are probably the major source, it is important to remember that the dust presently impacting the earth was probably generated by a number of different parent bodies. The collection of interplanetary dust is considered and a description is presented of the physical properties of interplanetary dust, taking into account the structure, elemental composition, and mineralogy.

Brownlee, D. E.↗
NASA NTRS2004

Meteoroid Engineering Model – Final Report

We developed a parametric model of the spatial distribution of sporadic meteoroids by taking their primary source to be short-period comets with aphelia less than 7 AU which showed to be the major source of dust released through the sublimation of cometary ices. We constrained the model to fit the observed distribution of particles deduced from the zodiacal light measurements of Helios I and II as well as Earth-based radio-meteor observations. We also considered the contribution to the sporadic meteor complex from long-period comets. An origin for the North and South Toroidal source of sporadic meteors is proposed and the results of calculations for the likely characteristics of asteroidal meteors are also presented. Predicted fluxes and radiant distributions of sporadic meteors in the region between Mercury and Mars are shown as well. We have also included the effects of the gravitational shielding and focusing of the planets and shown that flux enhancements of up to 70% are possible at some locations.

Jones, J.↗
NASA NTRS1974

Strongly differentiated material in high-inclination and retrograde orbits

Two definitive spectra of strongly differentiated meteors are discussed. The meteors entered the earth's atmosphere at velocities greater than 40 km/sec from high-inclination and retrograde orbits. Both were magnesium-calcium particles, and their spectra show them to be highly deficient in iron and sodium. The moon and the asteroids have been discussed as sources of such calcium-rich achondrites, but these sources give low or intermediate geocentric velocities. Long-period comets and meteoroids of probable cometary origin are the only objects now known to have orbits similar to the orbits of these two meteors.

Harvey, G. A.↗
NASA NTRS1977

Comets: Data, problems, and objectives

A highly abridged review of new relevant results from the observations of Comet Kohoutek is followed by an outline summary of our basic knowledge concerning comets, both subjects being confined to data related to the nature and origin of comets rather than the phenomena (for example, plasma phenomena are omitted). The discussion then centers on two likely places of cometary origin in the developing solar system, the proto-Uranus-Neptune region versus the much more distant fragmented interstellar cloud region, now frequented by comets of the Opik-Oort cloud. The Comet Kohoutek results add new insights, particularly with regard to the parent molecules and the nature of meteoric solids in comets, to restrict the range of the physical circumstances of comet formation. A few fundamental and outstanding questions are asked, and a plea made for unmanned missions to comets and asteroids in order to provide definitive answers as to the nature and origin of comets, asteroids, and the solar system generally.

Whipple, F. L.↗